SPACECRAFT REENTRY COMMUNICATIONS BLACKOUT

INTRODUCTION

When a spacecraft such as the Space Shuttle leaves orbit
and reenters the atmosphere as it travels to a landing site,
there is a critical period of time when all communications
between the spacecraft and ground are lost. This phenomenon
is due to the tremendous heating experienced by the craft
during reentry and is termed 'reentry blackout'.

NASA impression

In low Earth orbit the Space Shuttle or similar vehicle is
travelling at almost 8 km per second. To land safely on the ground this speed must be reduced to zero by making use of atmospheric drag. What NASA calls the 'entry interface' commences when the Shuttle descends to around km altitude. It does this by deorbit thruster firing. This rocket burn is
performed opposite to the direction of travel. The result
of this is a reduction of altitude rather than a reduction
in speed. The speed changes little due to an exchange of
gravitational potential energy for kinetic energy.

At around 120 km altitude, the atmospheric drag increases
significantly and the resultant heating increases as the
shuttle kinetic energy is exchanged for thermal energy.

A shockwave forms just in front of the nose and underside of
the spacecraft. Between this shock and the vehicle itself
temperatures may reach 10,000 to 12,000 Kelvin. (The heat
resistant surfaces of the shuttle only reach a maximum of
1600 K themselves.) This very high temperature ionises the
gas close to the shuttle forming a plasma cloud or miniature
ionosphere around the spacecraft. The plasma frequency
(that frequency below which radio communications is not
possible) may rise to many gigahertz around the lower parts
of the vehicle. This gives rise to a communication blackout
for direct communications between the Shuttle and ground
control. This typically lasts from 25 to 12 minutes prior
touchdown, a total outage of 12 to 13 minutes.

Maximum heating of the orbiter occurs during this time frame,
at an altitude of 70 km and about 20 minutes prior to
touchdown. Unfortunately this is also the most critical
time of reentry, and if any problems occur during this
phase of flight, the communications blackout prevents any
diagnostic telemetry from reaching the ground. Such was
the case with the catastrophic breach of the Space Shuttle
Colombia's hull during reentry on the first of February 2003.

The figure below illustrates a typically reentry, showing
the variation of altitude and velocity.

BLACKOUT PARAMETERS

The Saha equation may be used to calculate the ionisation
of the gas surrounding a deorbiting vehicle as a function of
temperature:

Ne = 5 x 1010 Na0.5 T0.75 exp(-5800 Vi / T)

where Ne is the electron density in electrons per
cubic metre, Na is the density of unionised atoms
or molecules, T is the plasma temperature and Vi
is the ionisation potential of the atom/molecule. This
formula is only strictly valid for less than 10% ionisation.

The plasma frequency is given by fp[Hz] =
9 Ne0.5

At an altitude of 70 km, where maximum Space Shuttle heating
occurs, the atmosphere is still about 20% oxygen and 80%
nitrogen. The Saha equation must be applied to these
constituents separately. When this is done, the results are
shown in both tabular and graphical form below.

T (Kelvin)

Ne /m3

fp

1500

1.7x103

370 Hz

2500

2.9x1011

4.8 MHz

3000

3.4x1013

53 MHz

4000

1.4x1016

1.1 GHz

5000

5.5x1017

6.7 GHz

6000

6.7x1018

23 GHz

7000

4.1x1019

58 GHz

8000

1.6x1020

115 GHz

9000

4.9x1020

200 GHz

high

2.0x1021

400 GHz

The Saha model becomes strictly invalid after 8000 Kelvin, but
the curve can be extended smoothly up to the point of complete first ionisation, when the number of ions/electrons equals the total atmospheric number density [ 2x1021 per cubic metre at 70 km ]. Of course, higher temperatures will result in more than one electron
being removed from the atoms, with a further increase in
plasma frequency, but we will ignore this in the current
approximation.

For a given sheath temperature, the frequencies above the
curve will propagate to and from the spacecraft, whereas
the frequencies below the line will not. For sheath
temperatures of 10,000 to 12,000 K, we see that a frequency
in excess of 350 GHz would be needed for direct Shuttle
communication with the ground at the time of maximum
heating.

There are however, ways in which communication is possible
during this critical reentry time. The sheath temperatures
on the top surface of the Shuttle are much less than those
on the bottom which, at a descent angle of about 22 degrees,
is where the shock is formed. Communications from the
topside via a satellite relay in the GHz range may be possible. Other options which have been tried are water
injection into the plasma, and also the use of very narrow
width pulsed signals, both with some success.